1. Field of the invention:
The present invention relates to a telescopic boom mechanism composed of a plurality of booms of different diameters telescopically assembled together and including two inner booms slidable into and out of the open ends of the boom of the maximum diameter for varying the distance between the distal ends of the inner booms.
2. Description of the Prior Art:
There have heretofore been used elevating apparatus for elevating a lifting table or platform to lift workers and/or materials to higher places for assembly, painting, repair in various locations such as construction site, highways, and other areas required work at elevated levels. Such conventional elevating apparatus include scissors-type lifts in the form of a pantograph comprising a plurality of vertically connected X-shaped arms with two arms in each X-shaped arm unit being centrally pivotally interconnected. However, in order to raise the lifting table to a higher position, the number of X-shaped arm units has to be increased. This has led to problems in that the lift as it is collapsed has an increased height, and workers will have difficulty in getting on and off the platform and also in loading and unloading materials onto and from the platform. To avoid such drawbacks, there has been proposed an elevating apparatus having an extensible and contractable arm assembly comprising a plurality of telescopic booms. Since however the booms, typically three in number, are inserted concentrically and slidable with respect to one another, there has been required a boom storing mechanism of special design.
FIG. 1 of the accompanying drawings illustrates a conventional telescopic boom mechanism including a hollow middle boom A with a hollow spacer B inserted coaxially therein, the middle boom A and the spacer B being interconnected at ends thereof. A lower boom C is slidably inserted between the middle boom A and the spacer B, and an upper boom D is slidably inserted in the spacer C. The lower and upper booms C, D can be moved in the directions of the arrows E, F, respectively, with respect to the middle boom A. When the boom mechanism is extended or contracted, the lower boom C slides in contact with the middle boom A and the spacer B, and the upper boom D slides in contact with the spacer B. Therefore, the lower and upper booms C, D can slide smoothly with respect to the middle boom A and the spacer B. With the prior arrangement, the spacer B is necessary to permit the lower and upper booms C, D to slide in isolation from each other. The need of the spacer B however complicates the machining and assembling of the mechanism, and increases the overall weight of the mechanism. Where the boom mechanism is employed in an elevating vehicle, the added weight reduces the operation efficiency of the elevating apparatus.